Reddit reviews Fm Theory and Applications: By Musicians for Musicians

You're basically asking about the fundamentals of FM (Frequency Modulation) synthesis (Operator is a FM synth, in case you didn't know). It's a very (like a book's worth deep) deep subject, but there are some decent enough tutorials to get you started – like this one.

I personally think it's the hardest method of "regular" synthesis methods to wrap your head around - it's not that grasping the basic idea is hard, but actually getting to the level where you can get to a sound you have in your head from scratch is much trickier than with traditional substractive stuff, simply because of its basic nature - everything affects everything else, more or less.

Lot's of useful information here, however Chowning literally wrote the book.

https://www.amazon.com/Fm-Theory-Applications-Musicians/dp/4636174828/